1. Field of the Invention
The present invention relates to a mounting structure of a magnetic member, a method of bonding the magnetic member and a magnetic coupling member, a pickup unit, and a disc apparatus.
2. Description of the Related Art
FIGS. 9A and 9B illustrate one embodiment of a mounting structure of a magnetic member and a method of bonding a magnetic member and a magnetic coupling member.
In an objective lens actuator for an optical pickup unit, high drive sensitivity is achieved by reducing the weight of a movable portion (not shown) of the actuator and increasing the volume of a magnet 550 disposed on a fixed portion 507 of the actuator to obtain large magnetic flux density from the magnet 550. An actuator means a driving apparatus that converts energy into translational motion or rotational motion, etc., for example. For the objective lens actuator, etc., for the optical pickup unit, a potting adhesive (not shown), which is cured by UV (Ultraviolet radiation) light, etc., is often used heavily to speed up an actuator assembling process, for example.
Other types of optical pickup units includes a moving magnet (MM) type optical pickup that enhances the adhesive strength between a lens holder and a yoke to reduce unnecessary vibrations, for example (see, e.g., Japanese Patent Application Laid-Open Publication No. H11-259877, pp. 1, 3 and 4, FIGS. 1 to 4). “Yoke” is considered as an object that reduces a leakage of magnetic force generated from a magnet, for example.
However, when the optical pickup unit or the actuator for the optical pickup unit accidentally drops and the actuator mounted with the magnet 550 receives an impact resulting from such a drop, the position deviation of the magnet 550 relative to a back yoke 510 may occur because of the weight/mass of the magnet 550. For this reason, strong adhesion is required between the back yoke 510 and the magnet 550.
When an ordinary potting adhesive which is cured by UV light, etc., is used on a bonding portion, however, improvement in adhesion may not be expected. When an adhesive surface 551 of the magnet 550 is fitted to an adhesive surface 511 of the back yoke 510 in a state where a UV-curing adhesive not shown is interposed between the adhesive surface 551 of the magnet 550 and the adhesive surface 511 of the back yoke 510, UV light is blocked by the magnet 550 and the back yoke 510 to be unable to reach the adhesive surfaces 551 and 511, which prevents the UV-curing adhesive from curing between the adhesive surfaces 551 and 511. Therefore, when the magnet 550 is bonded to the back yoke 510 using the UV-curing adhesive, the UV-curing potting adhesive is allowed to be applied only to the periphery of the magnet 550. This causes a problem that the UV-curing adhesive cannot be applied to the magnet 550 and the broad adhesive surface 511 of the back yoke 510.
The adhesive strength of the magnet 550 to the back yoke 510 may drop due to changes in humidity and temperature. When the magnet 550 is bonded to the back yoke 510 using the UV-curing adhesive, if the UV-curing potting adhesive is applied only to the periphery of the magnet 550 to be left cured thereon and, impact resulting from the drop of the actuator, etc., is given to the actuator mounted with the magnet 550 in such a situation, for example, the adhesion is not able to overcome a dropping force of the magnet 550 generated by its weight and mass, which causes another problem.
It is desirable, therefore, that an adhesive 560 be applied as widely as possible to the whole contact surface of the back yoke 510 to be mounted with the magnet 550, to bond the magnet 550 to the back yoke 510, thereby securing the adhesive strength of the magnet 550 to the back yoke 510. In this case, UV light does not reach the adhesive surface 551 of the magnet 550 and the adhesive surface 511 of the back yoke 510, so that the UV-curing adhesive not shown can not be used. When the magnet 550 is bonded to the back yoke 510 with the adhesive 560 in a state of being interposed between the adhesive surface 551 of the magnet 550 and the adhesive surface 511 of the back yoke 510, such an adhesive 560 is used that can cure under a lightless environment, e.g., one-component thermosetting adhesive 560, two-component mixed adhesive 560, and an anaerobic curing type.
It is preferable that the adhesive area of the adhesive surface 551 of the magnet 550 and the adhesive area of the adhesive surface 511 of the back yoke 510 be large in order to improve the adhesive strength of the magnet 550 to the back yoke 510. In order to apply the adhesive 560 to the adhesive surfaces 511 and 551 having large areas, it is desirable to apply the adhesive 560 in advance to the adhesive surfaces 511 of the back yoke 510 by a pre-application process, for example. However, when the adhesive 560 is applied in advance to the adhesive surfaces 511 of the back yoke 510 and then the magnet 550 is attached to the back yoke 510 coated with the adhesive 560, since the magnet 550 has the strong magnetic force, so that the magnet 550 is placed being sliding on the back yoke 510. At this time, the magnet 550 acts like a paddle against the adhesive 560, and thus, it may be possible for the magnet 550 to scrape off the adhesive 560 applied in advance to the back yoke 510.
In both cases of applying in advance the adhesive 560 to the back yoke 510 before an assembling process of placing the magnet 550 on the back yoke 510 and of applying the adhesive 560 to the back yoke 510 right after disposing the magnet 550 near the back yoke 510, the use of the permanently magnetized magnet 550 generating a strong magnetic force makes it difficult to confirm the magnet 550 is bonded to the back yoke 510 on a sufficiently large adhesive area.